1. Description of Related Art
The present invention relates generally to redundant data storage, and more particularly, to implementing RAID 1 mirroring by storage devices sharing a common bus.
2. Description of Related Art
Host adapter integrated circuits were widely used for interfacing two I/O buses such as a host computer I/O bus and a SCSI bus. Frequently, a host adapter integrated circuit was used in a data storage system that implemented redundant data storage.
Redundancy is increasingly becoming a requirement for data storage systems. If one data storage device fails, the data on the failed data storage device preferably can be reconstituted or reconstructed using data content on other data storage devices in the data storage system.
The simplest scheme for providing data redundancy was mirroring of data storage devices where data written to one data storage device was also written on another data storage device. This mirroring scheme was also referred to as RAID 1. With mirroring, if one data storage device failed, the data content was retrieved from the mirror data storage device. As disk drive prices have fallen, the mirroring scheme has increased in popularity.
One implementation of data mirroring used a software manager to identify two data storage devices, such as SCSI disk drives, and to issue write commands for the same data to the two data storage devices. The software manager generated two data write commands, which in the simplest case differed only in the target data storage device specified. This mirroring scheme was implemented only at the software manager level. The data storage devices, host adapters and the corresponding management software required no modifications to support this data mirroring technique. To these devices, the data mirroring was not evident because the devices were simply processing routine write commands.
While this data mirroring technique is easily implemented, the technique generates additional traffic on the I/O bus between the device executing the software manager and the host adapter for example. Frequently, this I/O bus is the busiest bus in the data storage system and so additional traffic on this I/O bus further exacerbates any I/O bottlenecks associated with this I/O bus. Also, since two commands are generated for each write, the memory in the system executing the software manager must have the capability to store the two commands as well as any memory structures used in monitoring the processing of the two commands. There is a similar requirement for reading mirrored data. Also, in addition to executing the software manager, the system processor must execute instructions to build and monitor the execution of the duplicate commands required for the mirrored transaction. Thus, while data redundancy is desirable, it adversely affects system performance in several different respects, and in some cases may prevent the implementation of mirrored transactions.